1. Field of the Invention
The present invention relates to a method for setting the flexibility of a manipulator according to changes in surroundings, and a device for controlling the flexibility of a manipulator.
2. Description of the Related Art
The flexibility of a manipulator or flexibility of an arm of a robot is an extremely important characteristic for a force control of the manipulator, which is an indispensable technique in manipulator operations with mechanical interactions between the manipulator and manipulating objects.
A manipulator possessing flexibility corresponding to the contents of the operations or external environments can perform difficult operations of, for example, an assembly operation of delicate parts or an operation in which the manipulator comes into contact with the human body, easily and safely compared to conventional highly rigid manipulators.
The flexibility of the manipulator means a characteristic where a manipulator changes the positions of its joints while providing a reaction against a resistance from a manipulating object, to the manipulating object, in the same way that a spring changes its length in accordance with a given force while generating a reaction.
The flexibility of a manipulator is referred to as mechanical impedance or compliance in the field of manipulator control development, and is defined by apparent inertia, viscosity, and elasticity of an effector with which the manipulator makes contact with the manipulating object.
When the manipulator performs the above operations, a dynamic interaction occurs between the manipulator and the manipulating object. Therefore, not only the position of the manipulator but also the force of the reaction must be controlled. As a typical force control method for a manipulator, the following two methods are used: a method in which the force is explicitly controlled using force feedback; and a method in which both the force and the position are controlled simultaneously by setting the flexibility or the mechanical impedance of the manipulator, which exhibits a relationship between the motion of the manipulator and the force between the manipulator and the manipulating object. The first method includes, for example, the hybrid control of the position and the force, and the second method includes, for example, the impedance control and the compliance control of the manipulator. The second method excels in adaptability to various kinds of objects and in stability for the modeling error of the manipulation object.
To date, some methods for setting the flexibility of a manipulator have been suggested in the following three references.
(1) "Compliance Setting Method" (Tetsu Matsuo and Bin Iwai, Japanese Patent application No. 03-143956, Japanese Laid-open Patent Publication No. 04-3431101), in which the requirements that the flexibility of a manipulator in a contact condition should satisfy are formularized as linear conditions, and the flexibility is solved as a question of linear programming using a proper objective function.
(2) "Iterative Learning of Impedance Parameters for Manipulator Control Using Neural Networks" (Toshio Tsuji, Masataka Nishida, and Kouji Ito, Transactions of the Society of Instrument and Control Engineers., Vol. 28, No. 12, 1461/1468, 1992.), in which the flexibility of a hand of a manipulator is described by an elastic matrix, a viscosity matrix, and an inertia matrix, using neural networks. According to this method, these matrices are set by performing a learning for desired trajectories of position and force. The target track means a track which satisfies a condition that the hand of the manipulator continues to push an object or a wall with a certain force of, for example, 1 N (newton).
(3) "On Control Design for Robot Compliant Manipulation" (Luo Zhi-wei and Masami Ito, Transactions of the Society of Instrument and Control Engineers., Vol. 26, No. 4, 427/434, 1990.), in which a reference model is designed by a model matching method in consideration of follow-up characteristics in free motion, compliance characteristics in contact performance, and adjustment characteristics for an environment.
However, the conventional methods for controlling the flexibility of manipulators have the following problems.
In conventional impedance control and compliance control, since the forces that a manipulator exerts on a object are decided by the relationship between the interaction and the motion of the manipulator indirectly, the flexibility of the manipulator must be set appropriately according to contents of the operation and characteristics of the object.
Traditionally, the flexibility of the manipulator is empirically set by individual operators. However, methods using the empirical knowledge of individual operators cannot deal with various kinds of operations and objects, and lack general versatility. The method for setting the flexibility of a manipulator is still in the development stage, and a method having high practicality is not yet available. Thus, establishment of a versatile and effective method by which appropriate flexibility is automatically set corresponding to the characteristics of the operations and the objects, is desired.